Electrocoagulation is a simple and effective method of generating metal hydroxide coagulants within water/wastewater by electro-oxidation of metals, such as iron and aluminum. Schematic and basic concept of electrocoagulation is presented in FIG. 19. When current is passed through the system, metal ions are released from a sacrificial anode, whereas hydroxide ions and hydrogen gas are produced at the cathode. The metal ions and hydroxide ions readily combine to form polymeric metal hydroxides, which are excellent coagulants.
Reactions that occur at the anode, cathode and bulk liquid are shown below with iron and/or aluminum as anode material.
At the anode:                Oxidation of metal:Fe(s)→Fe2++2e−Fe2+→Fe3++e−        Oxidation of water:2H2O→O2+4e−+4H+        
At the cathode:                Production of hydrogen gas and hydroxide ions:2H2O→H2+2OH−4e−+4H+→2H2         
In bulk liquid:                Formation of metal hydroxides:Fe2++2OH−→Fe(OH)2 Fe3++3OH−→Fe(OH)3         
For an aluminum anode, the corresponding oxidation of the metal will be:Al(s)→Al3++3e−and in bulk liquid the formation of metal hydroxide will be:Al3++3OH−→Al(OH)3 
Depending of the bulk liquid pH, mono- and polyhydroxides of the metals can be formed as follows:Fe(OH)2+, Fe(OH)2+, Fe(OH)2, Fe(OH)3, Fe(OH)63−, Fe(OH)4−,and when aluminum is used as anode material examples of formed hydroxides are:Al(OH)2+, Al(OH)3, Al(OH)4−, Al13(OH)345+, Al13(OH)327+
The metal hydroxides form flocculants and remove the contaminants by ion exchange, adsorption, neutralization and sweep-floc action. Metal cations neutralize negative surface charges of colloidal particles and pollutants. Then the particles approach closer and agglomerate under the influence of van der Waals forces. Besides various types of ions, organic substances may effectively be removed in the form of sediment flocs by electrocoagulation. Factors such as current density, pH, electrode type, reaction time, etc., play important roles in the formation of metal hydroxides. Generally, higher current density produces more metal ions in the solution. Likewise, solubility of the metal hydroxides largely depends on pH of the solution. Optimum pH range for the formation of iron and aluminum hydroxide flocculants is between 5 and 8. However, the flocs become soluble at lower or higher pH values. pH is equally important for the interaction of metal flocculants with pollutants. For example, removal of fluoride by aluminum hydroxide is thought to be an ion exchange process. Fluoride ion replaces one hydroxide ion from aluminum hydroxide and makes a complex according to the following reaction:Al(OH)3+xF−→Al(OH)(3−x)Fx(s)+OH−
The following equations provide theoretical relationships between fundamental characteristics of the electrocoagulation process applicable to the invented process and the invented device.